What type of reaction is a dehydration reaction?

A dehydration reaction is specifically characterized by the removal of a water molecule during the process of forming a larger molecule, typically from smaller subunits. In the context of biology, this reaction is crucial for building polymers, such as carbohydrates, proteins, and nucleic acids. For example, when two monosaccharides come together to form a disaccharide, a water molecule is released as a byproduct.

In contrast, the other options describe different chemical processes. A reaction that breaks down polymers is known as hydrolysis, which involves the addition of water to break bonds in the polymer. Producing hydrogen gas is not relevant to dehydration reactions, which focus on the formation of larger molecules through the loss of water. Lastly, a reaction that forms two molecules by using water would also refer to hydrolysis, not dehydration, since it involves adding water rather than removing it.